This invention relates to a high withstand voltage Darlington transistor circuit in which the breakdown tolerance when the load is short-circuited is increased.
Recently, high power Darlington transistors have come into wide spread use for inverter circuits for AC motor control, control circuits for DC motor control and emergency power supply, etc. In particular, they are being used as semiconductor devices for switching large currents in high power circuits. These high withstand voltage Darlington transistors commonly must be able to conduct a current, when turned on, of 10 .ANG. to 1000 .ANG., and be able to withstand a voltage of 450 V to 1000 V, when turned off.
An example of a circuit, which uses such high withstand voltage Darlington transistors for large current switching, is shown in FIG. 1. Darlington transistor Q1 comprises Darlington-connected transistors Q11, Q12. As is shown, collector C and emitter E of Darlington transistor Q1 are connected in series with load 11, fuse 13 and power source 15 for controlling load 11 by turning on and off. In general, Darlington transistors also include stabilizing circuits, speed up diodes, and fast recovery diodes, etc., description of which has been omitted. Darlington transistor Q1 turns on when a current is supplied to its base B, and the emitter E and collector C are virtually short-circuited. When, on the other hand, current is not supplied, Darlington transistor Q1 is turned off, and the collector-emitter path remains substantially open.
Darlington transistor Q1 is required to not breakdown when load 11, for example, a motor, short-circuits, until fuse 13 blows. For example, in FIG. 1, if the power source voltage Vcc is taken to be 480 V (AC), in the forward safe operation area characteristic (particularly the load short circuit safe operation area characteristic) and at an operating temperature of (100.degree.-125.degree. C.), Darlington transistor Q1 is required not to breakdown for over 10 .mu.sec., when a voltage of 800 V (DC) is applied.
The breakdown tolerance in the load short circuit safe operating area is determined by the area of the collector and by the electrical power, as is disclosed in "High Voltage High Power Transistor Modules for 440 V AC Line Voltage Inverter Applications" IPEC, March 1983, Conf. REC. Vol. 1, pp 297-305. It is also known that this breakdown tolerance also depends on the collector resistance Rc of Darlington transistor Q1. Consequently, in the past, in order to increase the breakdown tolerance, the collector resistance Rc was increased. For example, as shown in FIG. 2, with a transistor that has a cross-sectional structure comprising an N type emitter 21, P type base 23, N type low concentration intrinsic region 25 and N type collector region 27; that is, a so-called NP.nu.N transistor, the intrinsic region 25 is thickened vertically and the collector resistance Rc is increased to increase the breakdown tolerance. However, the increase in collector resistance Rc results in a drop in the various forward characteristics of the transistor. The problems that occur, for example, are an increase in the saturation voltage Vce(sat) between the collector C and emitter E, an increase in the switching time and a drop in the collector peak current. A drop in any of the characteristics is undesirable in large current switching transistors. If the problem concerned only a drop in the saturation voltage Vce(sat) or collector peak current, this could be solved by increasing the emitter area of the transistor. However, with an increase in emitter area comes an increase in the total size of the chip. This results in an undesirable increase in the cost of the transistor. Also, even if the emitter area is increased, it is impossible to decrease the switching time. Accordingly, up till now, a large current switching Darlington transistor, which satisfied both the economic demands and the operating characteristics, has not been available.